1. Field of the Invention
The present invention relates generally to a direct-current (DC) brushless motor, and a polygon scanner and an image forming apparatus having the same, and more particularly to a DC brushless motor which rotates a rotor by generating a rotating magnetic field while switching conduction to windings fixed on a stator core securely disposed in correspondence to the rotatable rotor having permanent magnets fixed thereon, a polygon scanner which has the DC brushless motor for rotating a rotating body having a polygon mirror fixed thereon to scan a laser beam for writing data, and an image forming apparatus for writing an image carrier or a photosensitive drum through a laser beam to form an image on the image carrier in an electrophotographic system. The present invention has particular applications in electrophotographic image forming apparatus suitable for a copying machine, a printer, a facsimile machine, a combination of these machines, or the like.
2. Discussion of the Background
Electrophotographic image forming apparatus employing a laser writing system for use in a digital copying machine, a laser printer, a facsimile apparatus, a combination of these machine, or the like have become rapidly more pervasive because of their high performance including a high printing quality, high speed printing capability, low noise and so on, as well as because of a reduction in price. A polygon scanner, which is a component of the laser writing system for these machines, is required to have the capability of rotating at an appropriate rotational speed corresponding to an image forming speed and a pixel density of the higher performance image forming apparatus.
Particularly, with an increasingly higher image forming speed and pixel density, the polygon scanner in the laser writing system is required to provide a high rotational speed exceeding 20,000 revolutions per minute, so that some conventional polygon scanners of a ball bearing type are not sufficient to satisfy a required quality in regard to an effective life of bearings, noise caused by the bearings, or the like.
For this reason, a polygon scanner employing a dynamic pressure air bearing has been proposed for higher rotational speeds by the same inventors as the present application (see for example Laid-open Japanese Patent Application No. 11-38346).
With such a polygon scanner having the capability of rotating at a higher rotational speed, power consumption is increased as the rotational speed is higher. Since a difference in efficiency between employed motors as driving power sources for polygon scanners noticeably appears in the difference in power consumption between the motors, a reduction in power consumption through an improvement in motor efficiency has been a critical issue.
For example, a DC brushless motor of a so-called radial gap inner rotor type is known (see Laid-open Japanese Patent Application No. 8-149775). Specifically, this type of DC brushless motor is composed of a rotor having permanent magnets fixed thereon, a stator core disposed outside the rotor with a predetermined spacing therebetween, a plurality of windings wound around the stator cores, and so on. A rotating magnetic field is generated by switching conduction to the windings to rotate the rotor.
Conventionally known stator cores used in DC brushless motors of this type may be classified into an open slot type, a half-open slot type, and a closed slot type. The open slot type stator rotor is described, for example, in Kokichi Ohkawa xe2x80x9cPermanent Magnet Motor,xe2x80x9d p185, published by Sogo Denshi Shuppan, 1975.
Since the closed slot type involves difficulties in a winding method and hence a higher manufacturing cost, the open slot type and the half-open slot type are generally considered more convenient due to their relatively easy winding operations.
In recent years, however, there is a tendency of preferentially manufacturing DC brushless motors of a so-called radial gap outer rotor type, which provide a higher production efficiency in a winding operation step than the radial gap inner rotors that employ the open slot type or half-open slot type stator rotor.
This is because the stator rotor used in the radial gap outer rotor type DC brushless motor has an open slot formed in an outer peripheral portion so that winding can be made more easily than the radial gap inner rotor type, which has an open slot in an inner peripheral portion.
In recent years, DC brushless motors of the radial gap outer rotor type have been widespread, and accordingly, a manufacturing cost thereof has been also reduced.
U.S. Pat. No. 5,382,853 discloses such a DC brushless motor of the radial gap outer rotor type, which includes a permanent magnet having four magnetized poles, six pole shoes and six windings.
Referring now to FIG. 1, which illustrates a conventional DC brushless motor 200, a permanent magnet 201 has four poles formed of two pairs of two polarities, and is rotatably supported by a rotor 202. A stator core 203 is disposed inside the permanent magnet 201 concentrically therewith.
The stator core 203, made of a ferromagnetic material, is formed with six T-shaped pole shoes 203a, each of which is wound with a winding 204. That is, six windings 204 are wound around the six pole shoes 203a. 
The windings 204 include three phases designated as a U-phase, a V-phase and a W-phase in FIG. 1, where a set of two windings U1, U2 form the U-phase; a set of two windings V1 , V2 form the V-phase; and a set of two windings W1, W2 form the W-phase.
A rotating position detecting mechanism 206 includes three rotating position detector elements 206a, 206b, 206c disposed at intervals of 60xc2x0, which generate rotating position detecting signals that are used by a driver circuit 205 (see FIG. 3) to switch conduction such that two phases are selected for conduction.
When the three rotating position detector elements 206a, 206b, 206c of the rotating position detecting mechanism 206 detect N, S, N poles, respectively, two phases, U-phase and V-phase, are selected and energized.
A current flows into the windings from the U1-phase and out of the V1-phase, causing the T-shaped pole shoe 203a wound with the U1-phase and U2-phase to have the S-polarity;
and the T-shaped pole shoe 203a wound with the V1-phase and V2-phase to have the N-polarity. Consequently, a magnetic repellent force or a magnetic attractive force acts between the permanent magnet 201 and the stator core 203 to rotate the permanent magnet 201 in the counter-clockwise direction as indicated by an arrow A in FIG. 1.
Referring now to FIG. 2 to explain how the windings 204 are wound around the respective pole shoes 203a. Viewed from the permanent magnet 201, U1 and U2 in the U-phase of the winding 204 are wound in the same direction and connected to each other such that a current conducted therethrough causes the T-shaped shoe poles 203a, wound with windings U1, U2, to have the same magnetic polarity (see again FIG. 1).
Similarly, V1 and V2 in the V-phase of the winding 204 and W1 and W2 in the W-phase of the winding 204 are wound in the same direction and connected to each other.
Referring next to FIG. 3, three winding groups 207 including the three sets of U-phase, V-phase, and W-phase windings 204 are connected in a Y-shaped connection configuration as generally indicated by reference numeral 208. Each of the three U-phase, V-phase, and W-phase windings 204 in the groups 207 has one end connected to an associated driver circuit 205, which switches the phases of the conducted windings 204 in accordance with rotating position detecting signals of the rotating position detecting mechanism 206, not shown in FIG. 3.
Referring next to FIGS. 4A and 4B, a description will be made on how the conduction is switched on the basis of a rotating position a detection made by the three rotating position detector elements 206a, 206b, 206c of the rotating position detecting mechanism 206 and the driver circuits 205 (FIG. 3) in response to the rotating position detection, as well as on a rotating magnetic field generated corresponding to switched conduction and the rotation induced by the rotating magnetic field of the rotor 202 having the permanent magnet 201 fixed thereon.
Specifically, FIGS. 4A and 4B show that the phases subjected to conduction are switched every 30xc2x0 to generate a rotating magnetic field which causes the rotor 202 having the permanent magnet 201 fixed thereon to rotate in the counter-clockwise direction as indicated by an arrow A in FIG. 1.
As the rotor 202 is rotated over an angular distance of 180xc2x0, the conduction is switched six times by the driver circuits 205, shown in FIG. 3, so that the conduction is switched twelve times during a full rotation of the rotor 202.
In the conventional DC brushless motor, as well as a polygon scanner and an image forming apparatus employing this DC brushless motor, however, magnetic circuits passing through the stator core are formed between the U1-phase and V1-phase windings and between the U2-phase and V2-phase windings of the DC brushless motor, respectively. Therefore, magnetic flux concentrates on two regions around the magnetic circuits, so that driving torques acting on the permanent magnets fixed on the rotor also concentrate on the two regions, thereby limiting effective utilization of magnetic forces generated by the permanent magnet over the entire periphery of the rotor. Particularly, the DC brushless motor suffers from a low rotation driving efficiency at a higher rotational speed due to an air flow loss, a switching loss, and so on, so that the DC brushless motor disadvantageously requires a high power consumption and a large size to compensate for the low efficiency.
The present invention has been made to solve the problems as mentioned above. In particular, it is an object of the present invention to provide a small direct-current brushless motor which is capable of realizing a variety of operational improvements including: an easy winding operation; a flat shape; effective utilization of a magnetic force generated by a permanent magnet fixed on the rotor over the entire periphery thereof; improved magnetic characteristics of a stator core; and a good rotation driving efficiency particularly at a higher rotational speed region.
It is another object of the present invention to provide a polygon scanner and an image forming apparatus which employ the improved direct-current brushless motor as described above.
To achieve the above and other objects, the present invention provides a direct-current brushless motor for switching conduction to windings wound around a stator core fixedly disposed corresponding to a rotatable rotor on which a permanent magnet is fixed, to generate a rotating magnetic field which causes the rotor to rotate. The direct-current brushless motor includes a permanent magnet having n magnetized poles, where n is an even number, a rotatably held rotor on which the permanent magnet is fixed, a stator core having T-shaped pole shoes corresponding to the magnetic poles of the permanent magnet, windings fixed on the stator core and including a plurality of phases for selectively generating magnetic polarities in sequence, a driver circuit for switching conduction to the windings, and a rotating position detecting mechanism for detecting a rotating position of the permanent magnet. The windings are divided into a plurality of winding groups for generating magnetic polarities in a plurality of the windings so as to produce a magnetic repellent force or a magnetic attractive force between all of the magnetic poles of the permanent magnet and T-shaped pole shoes of the stator core associated with selected phases by the driver circuit based on a rotating position detected by the rotating position detecting mechanism.
In addition, the stator core may include open slots formed between the T-shaped pole shoes on the outer periphery. The plurality of winding groups may include the number n of windings per phase, and are disposed on the circumference at equal intervals, corresponding to the phases being connected in a Y-shaped connection configuration. Adjacent windings in each of the phases are wound such that opposite polarities occur in alternation when a current flows therethrough. Thus, a current flowing into the winding groups from a common connection point of the Y-shaped connection configuration causes opposite polarities to occur in adjacent windings.
More specifically, the plurality of winding groups may include four windings per phase, and are disposed on the circumference at equal intervals, corresponding to the phases being connected in a Y-shaped connection configuration. Adjacent windings in each of the phases are wound such that opposite polarities occur in alternation when a current flows therethrough. Thus, a current flowing into the winding groups from a common connection point of the Y-shaped connection configuration causes opposite polarities to occur in adjacent windings.
The rotor may have an integrally fixed shaft radially and axially rotatably supported by a radial dynamic pressure air bearing and an axial bearing.
The permanent magnet fixed on the rotor may be disposed in a circumferential direction of the stator core. Alternatively, the permanent magnet fixed on the rotor may be disposed outside or inside the stator core.
Also, the stator core may be formed with a cylindrical inner surface opposite to the permanent magnet.
The direct current brushless motor may further include an annular member made of a ferromagnetic material and disposed along the outer periphery of the windings.
The driver circuit may selectively conduct windings in one or two of three phases of the winding groups to generate a rotating magnetic field, which causes the rotor to rotate.
In another aspect, the present invention provides a polygon scanner for driving a rotating body having a polygon mirror fixed thereon to scan a writing beam. The polygon scanner includes a polygon mirror, a rotor having the polygon mirror fixed thereon, and the foregoing direct-current brushless motor.
In a further aspect, the present invention provides an image forming apparatus for irradiating laser onto a photosensitive image carrier to write data thereon in an electrophotographic system. The image forming apparatus includes a rotatably held photosensitive image carrier, a charging mechanism for uniformly charging a surface of the image carrier, an exposing mechanism for irradiating laser onto the surface of the image carrier charged by the charging mechanism to form a latent image on the surface. The exposing mechanism includes the foregoing polygon scanner equipped with the direct-current brushless motor. Also included is a developing mechanism for developing the latent image formed by the exposing mechanism.
In the direct-current brushless motor of the present invention, a magnetic repellent force or a magnetic attractive force is produced between all of the magnetic poles of the permanent magnet and the T-shaped pole shoes of the stator core associated with selected phases by the driving mechanism based on a rotating position detected by the rotating position detecting mechanism, thereby making it possible to effectively utilize the magnetic force of the permanent magnet over the entire periphery, to improve the rotation driving efficiency at higher rotational speeds, and to reduce power consumption and the size of the direct-current brushless motor.
Also, the open slots formed between the T-shaped pole shoes on the outer periphery of the stator core facilitate a winding operation and hence contribute to a reduction in cost caused.
Specifically, the plurality of winding groups include n windings per phase, and are disposed around the circumference at equal intervals, where the winding groups corresponding to the phases are connected in a Y-shaped connection configuration. Adjacent windings in each of the phases are wound such that opposite polarities occur in alternation when a current flows therethrough. A current flowing into the winding groups from a common connection point of the Y-shaped connection configuration causes opposite polarities to occur in mutually adjacent windings. Consequently, the direct-current brushless motor of the present invention is advantageous in effectively utilizing the magnetic force of the permanent magnet over the entire periphery, improving the rotation driving efficiency at a high rotational speed, reducing power consumption, and reducing the size of the direct-current brushless motor. More specifically, the number of windings per phase may be four in the plurality of winding groups.
Further, the rotor may have an integrally fixed shaft which is radially and axially rotatably supported by a radial dynamic pressure air bearing and an axial bearing.
The permanent magnet fixed on the rotor may be disposed in a circumferential direction of the stator core. Alternatively, the permanent magnet fixed on the rotor is disposed outside or inside the stator core. The structure of the permanent magnet as mentioned contributes to a flat shape of the direct-current brushless motor and reduced vibrations of the rotor.
The stator core may be formed with a cylindrical inner surface opposite to the permanent magnet, thereby facilitating a winding operation and contributing to a reduction in cost.
The direct-current brushless motor may further include an annular member made of a ferromagnetic material extending along the outer periphery of the windings. With the annular member, the direct-current brushless motor can further improve the magnetic characteristic of the stator core, and hence more effectively utilize a magnetic force of the permanent magnet.
The driver circuit may selectively conduct windings in one or two of three phases of the winding groups to generate a rotating magnetic field which causes the rotor to rotate, thereby further improving the utilization factor of a magnetic force of the permanent magnet, which results in more effective utilization of the magnetic force over the entire periphery, a higher rotation driving efficiency at high rotational speeds, and more reduced power consumption.
In the polygon scanner according to the present invention, the rotor on which the polygon mirror is fixed, is rotated by the direct-current brushless motor of the present invention, so that the polygon scanner can provide a variety of advantages described above.
In the image forming apparatus according to the present invention, the surface of the rotatably held image carrier is uniformly charged by the charging mechanism, and laser writing is performed by the exposing mechanism equipped with the direct-current brushless motor and the polygon scanner of the present invention to form a latent image, which is developed by the developing mechanism to form an actual image, so that the image forming apparatus can also provide a variety of advantages described above.